A Vertical Cavity Surface Emitting Laser (hereinafter may be referred to as “VCSEL”) is a semiconductor laser that emits light in the direction perpendicular to its substrate. When compared with edge emitting semiconductor lasers, the VCSEL has some advantages including (1) lower cost, (2) lower energy consumption, (3) smaller sizes, and (4) easier to perform two-dimensional integration. Recently, because of the advantages, the VCSEL has attracted increasing attention.
The surface emitting laser has a current confined structure to enhance current influx efficiency. To form the current confined structure, a selective oxidation process is usually performed with respect to an AlAs (Al: aluminum, As: arsenic) layer. In the following, the current confined structure may be referred to as an “oxide-confined structure” for convenience (see, for example, Patent Document 1). The oxide-confined structure may be formed by forming a mesa structure having prescribed sizes and having a side surface on which a selectively-oxidized layer is exposed. Then, the formed mesa structure is processed under water-vapor atmosphere so that aluminum (Al) in the selectively-oxidized layer is selectively oxidized from the surface side of the mesa structure. By doing this, an unoxidized region remains at and near the center of the mesa structure. The unoxidized region (hereinafter referred to as a “confined region” for explanation purposes) becomes a passing region (or a “current injection region”) through which a driving current for the surface emitting laser passes.
The refractive index of the aluminum-oxidized layer (AlxOy) (hereinafter referred to as an “oxidized layer”) in the oxide-confined structure is about 1.6, which is lower than that of semiconductor layers. Because of this feature, a refractive index difference is generated in the lateral direction in the resonator structure of the surface emitting laser, and the light is confined in the center of the mesa structure, thereby improving the emission efficiency of the surface emitting laser. As a result, it becomes possible to obtain excellent characteristics such as lower threshold current and higher efficiency.
To further improve the emission efficiency of the surface emitting laser, it is effective to reduce the scattering loss of the lights by the oxidized layer. To that end, the oxidized layer may be positioned at a node of standing wave distribution of the electric field of the light (as described in, for example, Non Patent Document 1).
Further, in many applications of the surface emitting lasers, there is a strong demand for a beam having higher power and a single peak shape. However, unfortunately, in surface emitting lasers having an oxide-confined structure, due to a large refractive index difference in the lateral direction caused by the oxidized layer, an even higher-order lateral mode may also be confined and oscillated. To reduce the light confinement of the higher-order lateral mode, it is effective to reduce the refractive index difference in the lateral direction and reduce the area (size) of the confined region.
By positioning the oxidized layer at a node position of the standing wave distribution of the electric field of light, it becomes possible to reduce the influence of the oxidized layer to electricity distribution and also reduce the refractive index difference. Further, by reducing the area (size) of the confined region, a higher-order lateral mode having wider mode distribution may leak from the confined region; therefore, the confining effect with respect to the higher-order lateral mode may be reduced. Though it depends on the wavelength range, to realize a single fundamental mode oscillation, it is considered that the one side or the diameter of the confined region is required to be reduced to as small as three or four times the oscillation wavelength. For example, when the oscillation wavelength is 0.85 μm, the one side or the diameter of the confined region is 3.5 μm or less, and when the oscillation wavelength is 1.3 μm, the one side or the diameter of the confined region is 5 μm or less. By having this, simultaneously, the threshold current value becomes smaller.
However, when the size of the confined region is reduced as described above, a single fundamental mode may be controlled only when an injection level of the carriers is relatively low. Further, when the injection level of the carriers is relatively high, a higher-order lateral mode may be oscillated by the thermal lens effect caused by generated heat, or by the spatial hole burning. Especially, as described above, when the size of the confined region is reduced, the size of the oscillation region becomes accordingly smaller, which makes it difficult to obtain high power and makes the resistance of the surface emitting laser larger.
To overcome the problems and to respond to the demand for increasing the output power, there have been proposed several mode control mechanisms that may be used for surface emitting lasers and that do not depend on the oxidized layer.
For example, Patent Document 2 discloses a surface emitting semiconductor laser in which the diameter of the opening and the diameter of the current confined section are determined so that the difference between the optical loss in the oscillator in a high-order lateral mode of a laser light and the optical loss in the oscillator in a fundamental lateral mode of a laser light becomes larger based on the refractive index of the oscillator of the region with respect to the p-side electrode.
Further, Patent Document 3 discloses a surface emitting semiconductor laser in which a GaAs layer having a thickness indicating a high refractive index with respect to the oscillation wavelength is formed on an upper DBR mirror, and a groove is formed on the GaAs layer so that the groove is located above a dividing line between the Al oxidized layer and the AlAs layer, the groove having such a depth that the GaAs layer under the groove has a depth indicating a lower refractive index with respect to the oscillation wavelength.
However, unfortunately, in the surface emitting laser disclosed in Patent Document 2, the lateral mode characteristics, the output, and the like are extremely susceptible to the size of an electrode opening, the displacement between the electrode aperture and the selected oxidation structure, and the like. Because of the disadvantage, high alignment accuracy and high shape controllability for fabrication become necessary, which makes it difficult to uniformly manufacture surface emitting lasers. In addition, severe process control needs to be performed, which results in the increase of the manufacturing cost.
Further, the surface emitting laser disclosed in Patent Document 3 requires processes of forming a dielectric film and partially removing the dielectric film, which disadvantageously increases the manufacturing cost. Additionally, the device characteristics are susceptible to the accuracy of the displacement between the dielectric film and the current injection region, which makes it difficult to uniformly manufacture the surface emitting lasers.
On the other hand, when one of the plural low refractive index layers in a semiconductor multilayer film reflection mirror is entirely a selectively-oxidized layer (as described in, for example, Patent Document 1 and Patent Document 4), the thickness of the oxidized layer becomes in a range from 50 nm to 80 nm, which may cause large distortion due to volume shrinkage caused by the oxidation. The oxidized layer is disposed near the active layer because of the purpose of the oxidized layer. However, the oxidized layer may serve as a main component accelerating the degradation due to the distortion, and there is a tendency that the thicker the oxidized layer is, the faster the degradation proceeds.
Patent Document 4 discloses a surface emitting laser in which intermediate thin films are formed on both sides of the current confined layer. The intermediate thin films are AlGaAs thin films having a composition ratio of Al being 0.38 and having a thickness in a range between 20 nm and 30 nm.
However, in the surface emitting laser disclosed in Patent Document 4, all the low refractive index layers are oxidized. Therefore, the oxidized layer becomes thicker and the distortion due to volume shrinkage caused by the oxidation may negatively affect the active layer and accelerate the degradation of the characteristics. Further, in the surface emitting laser disclosed in Patent Document 1 and Patent Document 4, when viewed from the active layer, the current confined layer is located between the node and the antinode positions of the electric field intensity distribution, which disadvantageously increases the diffraction loss and reduces the single mode output.
Patent Document 5 discloses an oxide-confined VCSEL including a distributed Bragg reflector having a heavily-doped high Al content (for example, 95% or more, and preferably about 98%) oxide aperture forming layer provided between a low Al content (for example, between 0% and 35%, and beneficially about 15%) first layer and medium Al content (for example, around 65%, and preferably less than 85%) second layer. Further, between the first layer and the oxide aperture forming layer, there is provided a transition layer which is a relatively thin layer having a thickness of about 20 nm. In the transition layer, Al concentration linearly changes across the thickness.
On the other hand, in a so-called composition gradient layer for reducing the electric resistance in the semiconductor distributed Bragg reflector, it is preferable to selectively increase the doping (see, for example, Patent Document 6). Further, preferably, the composition gradient layer is located at a node position of the electric field intensity distribution to avoid the increase of the absorption loss. Further, preferably, the oxide-confined structure is located at a node position of the electric field intensity distribution to reduce the diffraction loss.
However, unfortunately, in the oxide-confined VCSEL disclosed in Patent Document 5, the oxide aperture forming layer (corresponding to the current confined structure) and the transition layer (corresponding to the composition gradient layer) adjoin each other. Therefore, it is difficult to locate both of the layers at a node position of the electric field intensity distribution at the same time.
The oxidation rate of the selectively-oxidized layer including Al and As is susceptible to the film thickness, composition rate of Al and As, oxidation temperature, and the like (see, for example, Non Patent Document 2). Further, the oxidation rate of the selectively-oxidized layer is influenced by the thickness of the natural oxidation film that has been formed on the side surface of the selectively-oxidized layer just before the oxidation process starts.
When the oxidized amount is different from that desired and accordingly the size of the current injection region varies, the size of the region that contributes the oscillation in the active layer may vary. As a result, the device characteristics including the light output may vary and the yield of the product is reduced. Especially, the size of the current injection region of single-mode devices is smaller than that of the multi-mode devices. Therefore, the device characteristics of the single-mode devices are likely to be more severely affected by the variation of the oxidation amount in the selectively-oxidized layer. Especially, when the size of the current injection region becomes larger than desired, the device may be operated in multi-mode and the yield of manufacturing single-mode devices is disadvantageously reduced.    Patent Document 1: U.S. Pat. No. 5,493,577    Patent Document 2: Japanese Patent Application Publication No. 2002-208755    Patent Document 3: Japanese Patent Application Publication No. 2003-115634    Patent Document 4: Japanese Patent Application Publication No. H11-26879    Patent Document 5: Japanese Patent Application Publication No. 2006-504281    Patent Document 6: Japanese Patent No. 2757633    Non Patent Document 1: A. E. Bond, P. D. Dapkus, J. D. O'Brien, “Design of Low-Loss Single-Mode Vertical-Cavity Surface-Emitting Lasers”, IEEE Journal of selected topics in quantum electronics, vol. 5, No. 3, pp. 574-581, 1999.    Non Patent Document 2: J. Select, “Topics Quantum Electron”, vol. 3, pp. 916-926, 1997.
According to research on the optical characteristics of many conventional surface emitting lasers, the optical characteristics such as single-mode output may vary even when the size of the current passage regions are substantially the same. Further experiments were performed and it was found that the thickness of the oxidized layers may vary between lots and even in the same lot even when the thickness of the selectively-oxidized layer, the Al composition, and oxidation conditions are set constant; the variation of the thickness of the oxidized layer is one of the causes of the variation in the optical characteristics; and the thickness of the oxidized layer remarkably varies especially on the side surface of the mesa where the oxidation starts. In addition, some surface emitting lasers show that the thickness of the oxidized layer from the oxidation start section (side surface of the mesa) to the oxidation end section (middle-inside part of the mesa) are uneven.